The present invention relates generally to D/A (digital to analog) converter systems useful for reproducing analog signals from digital data and more particularly to improvements in such systems for minimizing noise attributable to sampling and quantization errors. Thus, these improvements eliminate or reduce the need for a low-pass filter to reduce the sampling noise.
It is common practice to digitally sample an analog signal, e.g., representative of speech, sound synthesis, or video, to generate digital data samples that can later be used to reproduce a copy of the original analog signal. Digital sampling is desirable since digital signals can be reproduced with essentially no errors, thus providing improved reproduction over analog techniques. This digital data can be stored using any process available to data processors, e.g., in digital memory, on a hard or floppy disk, on CD-ROM, etc.
To reproduce an analog signal via digital sampling, the sampling rate must be at least two times the bandwidth of the original analog signal. This rate is referred to as the Nyquist rate. Digital sampling introduces two types of errors. The first type of error is due to the sampling rate and generates noise at the sampling frequency and at multiple harmonics of the sampling frequency. For example, an audio signal with an original bandwidth of 4 KHZ and sampled at an 8 KHZ Nyquist rate will have sampling noise at 8 KHZ, 16 KHZ, 24 KHZ, 32 KHZ, etc., in the reproduced signal. Thus, it is desirable to low-pass filter the reproduced signal to eliminate this sampling noise. However, while an ideal low-pass filter can eliminate this sampling noise, real world low-pass filters may either partially limit the original signal or permit some of the sampling noise to survive. Alternatively, increasing the sampling frequency would simplify the task of the low-pass filter. However, the quantity of digital data would also increase significantly.
Additionally, a digital sample generally can not identically represent the original analog signal even at each sampling point due to quantization errors, the inability of a digital representation with a limited number of data bits to accurately represent the original analog signal. While increasing the number of data bits could decrease this error, such a change would also increase the digital data storage requirements and in the case of communication systems increase the required transmission bandwidth.
JP. Pat No. 1-215126 to Takeda and U.S. Pat. No. 3,754,236 to Dotson, Jr. disclose circuitry that generate multiple delayed digital source signals, convert the delayed digital signals into analog signals with multiple D/A converters and then manipulate the converted analog signals to provide final interpolated signals. JP. Pat. 59-36422 to Katou discloses a similar concept to Takeda except it applies this concept to PWM (Pulse Width Modulation).